This invention relates to a measurement apparatus and method for determining the electronic state of a substance.
Photoemission spectroscopy is well known as a method of determining the electronic state of a substance. In photoemission spectroscopy, external light is made to impinge upon a sample and the kinetic energy, momentum, electron spin, etc., of electrons emitted by the sample are measured. This method is suited mainly for investigating occupied electronic states.
As a method of investigating unoccupied electronic states, a method of detecting light or an electron beam from the occupied state by excitation is known. With this method, however, it is difficult to independently obtain information relating to the unoccupied state owing to the influence of the density of the coupling states of the occupied and unoccupied electronic states and the Coulomb interactions between electrons and positive holes. Inverse photoemission spectroscopy has appeared in recent years and is an effective expedient for investigating unoccupied electronic states.
The following phenomenon is utilized in inverse photoemission spectroscopy: When an electron beam of monochromatic energy Ei impinges upon a sample from the outside (vacuum), the electron beam undergoes an electric dipole transition to an unoccupied electronic state of lower energy. Light is emitted at such time. By measuring the emitted light, the unoccupied electronic state can be determined.
Two types of measurement apparatus are known for practicing this inverse photoemission spectroscopy. One type of apparatus is so adapted that the emission strength of light having a constant energy of h.nu. is measured as a function of the energy Ei of the incident electron beam. The apparatus of this type employs a bandpass-type photodetector. The second type of apparatus is of the spectroscope type and is adapted to measure an emission spectrum distribution of the energy Ei of each incident electron.
Known examples of detectors for use in the first type of measurement apparatus include a detector which employs a Geiger-Muller counter, and a detector which is a combination of a Cu-BeO photomultiplier and a CaF.sub.2 or SrF.sub.2 entrance window.
The bandpass filter used in the detector which employs the Geiger-Muller counter comprises a combination of an entrance window and a gas. The entrance window, which consists of CaF.sub.2, for example, absorbs and cuts off light having a short wavelength. An example of the gas is I.sub.2 gas, which is used as an ionized gas (which may be mixed with a rare gas serving as a quench gas). The gas is for cutting off light in the long-wavelength region.
A detector which is a combination of a Cu-BeO photomultiplier and a CaF.sub.2 or SrF.sub.2 entrance window is described in Rev. Sci. Instrum., 58, 710, published in 1987 by I. Schafer, W. Drube, M. Schluter, G. Plagemann and M. Skibowski, and in the Journal of the Spectroscopical Society of Japan (Bunko Kenkyu), Vol. 39, No. 1, P. 6, published in 1990 by Shigemasa Suga, Hirofumi Namatake, Susumu Ogawa and Toyohiko Kinoshita.
This detector is constructed by providing, in the order mentioned, a light shield, an entrance window consisting of CaF.sub.2 or SrF.sub.2 and an electric-field shielding mesh in front of a Cu-BeO photomuliplier.
The bandpass characteristic of this detector is decided by the characteristics of the Cu-BeO photomuliplier and the characteristics of entrance window. The low-energy cutoff region of the bandpass characteristic is obtained by the first transition of the Cu-BeO photomultiplier sensitivity and cuts off long wavelengths. By way of example, the aforementioned sensitivity exhibits a characteristic which starts to rise from the vicinity of 7 eV and rises sharply in the neighborhood of from 8 eV to 9 eV.
On the other hand, the high-energy cutoff region of the bandpass characteristic is obtained by the sharp absorption edge of the entrance window and cuts off short wavelengths. By way of example, the absorption edges of CaF.sub.2 and SrF.sub.2 are 10 eV and 9.8 eV, respectively.
By combining these two, the peak of the bandpass characteristic appears in the vicinity of 9.8 eV and 9.4 eV.
The above-mentioned detector is disadvantageous in that it has a resolution of only 0.6 to 0.3 eV and possesses a low-energy cutoff characteristic which does not exhibit a steep rise. In addition, the limit on the full half-width of the bandpass characteristic is 0.3 eV. The reason for the foregoing is that low-energy cutoff is performed using the photoelectric emission threshold values of the gas and Cu-BeO.